Evaluation of non-linear valley effects through numerical modeling: the case of Norcia (Italy)

The dynamic non-linearity behavior affecting soils during earthquakes is a critical factor that can significantly influence seismic response, particularly when combined with site effects. Understanding this phenomenon is essential for seismic risk mitigation, especially in areas with active tectonics like the Central Apennines valleys, including the Norcia basin, which is the focus of this study. These flat areas often host industrial and historical urban centers, which increases the need for effective seismic risk reduction strategies.

This research aims to investigate the effects of non-linearity on site response in valley areas using both 1D and 2D dynamic numerical models along a section passing through Norcia city. Different seismic signals with increasing PGA values are used in modeling to test the model under varying levels of seismic stress and simulate the achievement of non-linear dynamic behavior. Two main goals are in fact pursued: to examine how site response varies depending on location within the basin and to investigate the effect of increasing Peak Ground Acceleration (PGA) on site response.

The first result of this work is been the creation of a robust geological subsurface model for the Norcia basin for subsequent numerical modeling. Previous studies carried out on the area show disagreements about the dominant site effects and the geological model of the subsurface. None of them provided a detailed discrimination of the seismic layers within the basin fill. After a review of previous data and elaboration of new data acquired, a calibration of the model using the Generalized Inversion Technique (GIT) is performed. This process has allowed to refine geometric and parametric details and defining a non-homogeneous basin. This process also led to a revised identification of the geological and seismic bedrock, emphasizing the importance of distinguishing between these two layers.

Twenty-five 2D numeric simulations are run, with the same number of different seismic inputs, selected from the Italian accelerometric database. The PGA values ranging from 0.03g to 0.36g. The medium frequency range of the signals (low, medium, and high) is also considered. Several control columns are extracted along the section and performed also in 1D simulations. More than 400 surface accelerograms are analyzed to obtain response spectra and compare 1D and 2D models trough the basin.

The results of the study are showed in terms of the aggravation factor (AG) and the Valley Amplification Factor (VAF) and allow a deeper understanding of their relationship with the PGA, aspect that recent definitions of these factors still do not account for.